1. Field of the Invention (Technical Field):
The present invention relates to methods and apparatuses for the production of methane from waste material such as manure.
2. Description of Related Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Millions of tons of solid wastes are generated each year from municipal, industrial, and agricultural sources. Also, the methane and carbon dioxide released from landfills is a major concern for global warming. There is a great deal of interest for alternative waste management techniques which can accelerate the anaerobic decomposition of the organic fraction of the solid waste.
There are various methods used for the stabilization of organic waste. These include composting, single phase anaerobic digestion, and two-phase anaerobic digestion. The composting causes quick decomposition but also results in uncontrolled release of carbon dioxide to the atmosphere without the potential benefit of capturing energy of the waste. In contrast, the single phase anaerobic digestion system can produce gas with 40-60 percent methane which can be put to beneficial use. However, in the traditional single phase anaerobic digestion system, the acid forming and methane forming bacteria exist in the same biological environment. In such an environment, the volatile fatty acids (“VFA's”) production proceeds at a much faster rate than the rate of conversion of VFA's to methane. This may cause acids to accumulate, resulting in a pH drop, and a consequent inhibition of methanogenesis.
Two-phase anaerobic digestion systems have been designed to avert the imbalance between the processes of acidogenesis and methanogenesis. (Pohland, F. G. and Ghosh, S., “Developments In Anaerobic Treatment Process”, Biotechnol. and Bioeng. 1971, 2, 85-106; U.S. Pat. No. 4,323,367; U.S. Pat. No. 4,396,402). The imbalance was removed by physically isolating the two major microbial phases in two separate reactors.
A two-phase system consists of a solid phase reactor and methane phase reactor. In the solid phase, water is applied from the top of the waste using a drip or sprinkler irrigation system. The leachate is collected at the bottom of the solid phase using an under-drain sump, and the leachate is then re-circulated through the solid waste bed until a desired level of volatile fatty acids (“VFA's”) is achieved in the leachate. At this point, the leachate is transferred to the methane production reactor where the VFA is converted to methane in a very short time (2-3 days). The overflow from the methane production reactor is then returned to the solid phase for recirculation through the solid waste bed to replenish the VFA concentration. The system works with a relatively small quantity of water (about 25% greater than the solid waste field capacity on weight basis) which is constantly re-circulated between solid phase and methane phase.
The two-phase system has several advantages over the traditional single phase systems. However, the two phase system is difficult to implement and costly to build. Therefore, its application has been limited.
Examples of other approaches include the use of tanks that are simply loaded with solid waste which is allowed to react with bacteria over time. (Wolff, J., and Appelfeldova, V., “Anaerobic fermentation of farmyard manure and the factors influencing biogas production”, Zemedelska Technica 1988, 3, 165-171; Sarapatka, B., “A study of biogas production during anaerobic fermentation of farmyard manure”, Biomass and Bioenergy 1993, 5, 387-393). However, such static containers suffer from mass transfer problems resulting from the presence of non-homogenous material and the lack of mixing, thus resulting in a low biogas yield. Also, the reactors described are heavy (metal) and large equipment is needed to remove the caps or covers of such reactors. The construction of such reactors is expensive.
Municipal solid wastes are major sources of air, water, and soil contamination. There is a need for alternative waste management techniques that are cost effective to better utilize the waste and minimize its adverse environmental impact.